Love the way you have spelled School
Mileage isnt that much of an issue i've done 60k in almost 8 years in my E class i'm pretty surewhen the car has long since gone to the great scrapyard in the sky the engine has a good chance of being ok.
Whilst you can change an engine its hard to exchange everything else becuase its rusted
That said its far from unheard of for E320 CDI engine to fail , i've heard of cracking cylinder heads and broken camshafts, diesels pumps, injectors issues , EGR valves etc etc
The 1.9 VAG engine is one of the best if not THE best and longlived engine on the market i dont see why they should be any less reliable depending on what badge is on the front
As for a barge i drive a E Class estate FFS, i'm in the market for a barge i have a fun car when needed , i want comfort and toys and somewhere nice to be